参数法
设x=2√2cosa-1,y=2√2sina
x+y=2√2cosa-1+2√2sina=2√2(cosa+sina)-1=4cos(a-π/4)-1
对于cos(a-π/4)属于[-1,1]
所以-4≤4cos(a-π/4)≤4
-5≤4cos(a-π/4)-1≤3
即-5≤x+y≤3